Thanks for all the advice, guys. FYI, the rotor was an M-B OE part that I installed. The other three rotors I installed at the same time are still working fine so I can't really blame the mechanic.
I called the shop I got the parts from and they told me that the rotor was still under warranty. They told me to bring the car by and they will diagnose the problem. If it is a bad rotor, they will replace it for me free (incl. labor!). If it's something else, I'll owe them for an hour of shop time plus any parts they replace. That doesn't sound like too bad a deal to me, so off to the shop I go!